As an X-ray imaging apparatus that aims at medical diagnosis, an X-ray photography system using an X-ray detector that includes an intensifying screen and film. In recent years, a digital X-ray imaging system that generates an image by digitally detecting X-rays has prevailed in place of such X-ray photography system. Typical one of digital X-ray imaging apparatuses acquires an X-ray image using a flat panel sensor as a detection device. In this apparatus, a solid-state imaging element which has sensitivity to X-rays and converts and outputs an electrical signal corresponding to the detected X-ray intensity, or a unit which combines a phosphor that absorbs X-ray energy and emits fluorescence accordingly, and a photoelectric conversion element that has sensitivity to visible light and converts it into an electrical signal corresponding to light intensity is used, and an analog signal from such element is A/D-converted into digital data, thus acquiring an X-ray image.
Such digital X-ray imaging apparatus comprises an examination module including a detection device which detects the quantity of electricity according to an X-ray transmitted dose, and converts it into a digital quantity, and a controller which controls this examination module and an X-ray generator. In general, a so-called X-ray imaging system is formed by combining this digital X-ray imaging apparatus, a monitor or printer that displays (outputs) a radiographic image, and an X-ray generator. In such digital X-ray imaging system, digital image data from the examination module is sent to a controller and undergoes various image processes, and an X-ray digital image to be diagnosed by a doctor is acquired using the processed image data. The generated digital image is output onto a film via a printer as needed or is displayed on a monitor to make a diagnosis after it is sent to a storage.
A general digital imaging apparatus will be described below with reference to FIG. 15. Referring to FIG. 15, reference numeral 1000 denotes an X-ray tube; 1001, an X-ray generator controller; 1002, an X-ray generator console as a console of the X-ray generator controller 1001; 1003, an imaging unit; 1004, an AEC device; 1005, an X-ray detector; 1007, an imaging controller in the X-ray imaging system; 1008, a display unit that displays an X-ray image; and 109, a patient. Note that the X-ray tube 1000, X-ray generator controller 1001, and console 1002 form an X-ray generator.
When a radiation generation signal transmitted from the controller 1007 of the X-ray imaging system is ON, the X-ray generator controller 1001 controls the X-ray tube 1000 to generate radiation. The radiation generated by the X-ray tube 1000 is transmitted through the patient 109 as an object to be examined, and reaches the imaging unit 1003. At this time, the radiation is scattered and absorbed inside the body of the patient. Since only primary radiation that goes straight through the patient can be used in imaging, scattered radiation is not necessary for the X-ray detector 1005. Hence, a grid (not shown) is normally provided to remove unnecessary scattered radiation to improve the contrast of a radiographic image.
The grid (not shown) is a plate formed by cutting a lamination obtained by alternately laminating lead plates and aluminum plates, in a direction perpendicular to the layer direction, and removes unnecessary scattered radiation generated from the patient 109 by arranging such plates which are juxtaposed nearly parallelly in a direction that agrees with the primary radiation propagation direction. A radiographic image of the grid is recorded by the X-ray detector 1005 to be superimposed on a radiographic image which is transmitted through the patient 109. In this case, the grid image does not pose any problem in diagnosis by appropriately selecting the spatial frequency of the grid.
The patient 109 scatters and absorbs radiation, but the degrees of scattering and absorption depend on the structure of the patient 109. Simply put, a patient with a large body thickness has high degrees of scattering and absorption, but a patent with a small body thickness has low degrees of scattering and absorption. Even when the body thickness remains the same, the degrees of scattering and absorption change depending on a muscular or fatty body type. Furthermore, the degrees of scattering and absorption change depending on a portion to be radiographed. For this reason, it is difficult to accurately estimate primary radiation dose transmitted through the patient 109 before imaging.
Hence, the X-ray imaging apparatus normally has an AEC (Automatic Exposure Control) device (see Japanese Patent Laid-Open No. 2001-149359). The AEC device is also called a phototimer in Japan. The AEC device is provided to minimize exposure on a human body and to appropriately generate a radiation dose required for the X-ray imaging apparatus. In the imaging unit 1003, the grid (not shown), AEC device 1004, and X-ray detector 1005 are laid out in turn from the entrance side of radiation. The AEC device 1004 detects some rays of the radiation which has been transmitted through the patient 109 and grid in real time, and transmits them as an AEC signal to the X-ray generator controller 1001. When the integrated value of the AEC signal has exceeded a threshold value, the X-ray generator controller 1001 turns off the radiation generation signal to stop generation of radiation. In this manner, the AEC device 1004 appropriately controls the radiation dose upon X-ray imaging.
FIG. 16 is a view for explaining the positional relationship between the X-ray detector 1005 and AEC device 1004. The X-ray detector 1005 is formed by arranging 2688×2688 160-μm2 square pixels in a matrix. In this case, an AEC device 1004 having three regions is attached. Imaging is made by selectively using one or a plurality of the three AEC regions in accordance with a portion to be radiographed as needed. When a region of interest is located at the center (e.g., abdomen or head), the central region C is selected; in chest imaging having both lungs as regions of interest, the upper right and upper left regions B and A are selected. After the AEC region (or regions) is selected, positioning is made to irradiate the region of interest of the patient with required X-rays. For example, in chest PA imaging, the patent stands with his or her back to the X-ray tube 1000 and with his or her chin on the upper end of the imaging unit 1003. Also, the patient raises shoulders by lightly turning arms behind him or her so as to allow easy observation of the lung fields. After positioning of the patient is normally done, imaging conditions such as a tube voltage, tube current, and the like and the selected region (or regions) of the AEC device are confirmed to appropriately set the dose of the region of interest, thus making an imaging process.
Note that the AEC device 1004 takes various attentions so as not to influence the image quality and exposed dose while sufficiently effecting its function. The AEC device 1004 covers very small areas of the X-ray detector 1005, e.g., the upper right and left portions corresponding to the two lung portions and the central portion, so as not to increase the exposed dose in the AEC device. Also, the AEC device is designed not to influence X-ray imaging. By detecting the radiation doses of partial regions of characteristic lung fields, an appropriate density is set for the entire lung fields in the obtained radiographic image. Furthermore, when the AEC device 1004 is superimposed on a patient image, it becomes an artifact. Hence, the AEC device 1004 is designed to shield nearly no X-rays. For example, the AEC device 1004 is a flat air bath, the outer periphery of which is made up of a material that does not shield radiation as much as possible. By collecting slight charges ionized in this air bath due to radiation, the radiation dose is detected. Or the AEC device 1004 is a thin fluorescent screen, which detects fluorescence generated by radiation using a photointensifier. Furthermore, an incoming X-ray dose may be integrated using a portion of the X-ray detector 1005 to shield X-rays radiated from the X-ray tube 1000.
The X-ray detector 1005 detects a radiographic image obtained using an appropriate radiation dose, and the controller 1007 of the X-ray imaging system applies various image correction processes and image processes to obtain a diagnosis image, which is set before diagnosis.
However, in the aforementioned X-ray imaging apparatus, when the AEC regions (A, B, and C) are not appropriately selected, or when positioning of the patient is incorrect, AEC does not function normally and results in an insufficient dose, and the image graininess drops, thus adversely influencing diagnosis. In such case, the operator must select correct AEC regions again or must re-make positioning of the patient to perform an imaging process again. Conventionally, since the operator cannot immediately recognize any selection errors of AEC regions or any deviations of AEC regions from the regions of interest, it is difficult to perform an imaging process under appropriate X-ray dose control using AEC.
Especially, in case of a side imaging process of a body, individual differences are large, and it is difficult to determine the position of interest of a spine from outside due to a different body thickness. Hence, even when the imaging process is repeated, appropriate X-ray dose control cannot be done based on correct AEC.